1. Field of the Invention
The present invention relates to a semiconductor laser which can be used as a light source of an optical communication apparatus and, more particularly, to a structure of a semiconductor laser which is suitably used to improve emission efficiency and a modulation band.
2. Description of the Background Art
With the recent development of information technology, demands for transmitting a large amount of information more rapidly and more distantly are rising. In order to reply to the demands, an optical communication technique using an optical fiber is developed, and is popularly used at the present. For example, in a semiconductor laser serving as a light source which is a key device in a communication apparatus, a large number of techniques for improving emission efficiency and high-speed response (i.e. a large modulation band) are used. More specifically, the semiconductor laser has an active layer which employs a multiple quantum well structure or a modulation-doped structure. With such techniques, as much information as possible can be transmitted at short intervals.
FIG. 4 is a conceptual energy band diagram of a multiple quantum well structure in an active layer of a semiconductor laser. The principle of emission of a laser having the illustrated structure will be described below. On a valence band side 21, holes 22 are injected from a p-type cladding layer side 2 into respective valence band well layers 23 through a light confinement layer 3. On conduction band side 26, electrons 27 are implanted from an n-type cladding layer side 6 into the conduction band well layer. Holes 22 and electrons 27 are combined to each other to generate light 28.
The laser shown in FIG. 4 has a multiple quantum well active layer 4 constituted by a plurality of quantum wells. With this structure, concentrations of holes 22 and electrons 27 become high, and an optical gain increases. According to this reason, emission efficiency is improved. At the same time, since a differential gain also increases, a relaxation oscillation frequency which expresses the modulation band of the laser also increases.
As another means for improving emission efficiency and modulation band, a technique is modulation-doped in which p-type or n-type doping of only the barrier layer 24 of multiple quantum wells of an active layer is provided. This technique is introduced to JAPANESE JOURNAL OF APPLIED PHYSICS VOL. 29, (1990) 81. According to this reference, when p-type doping is performed to barrier layer 24 of the multiple quantum well, gain spectrum is narrowed by a large number of holes 22 supplied from an acceptor level. Therefore, the differential gain increases, and the modulation band is improved. On the other hand, when n-type doping is performed, optical absorption is suppressed. Therefore, a threshold current is reduced, and emission efficiency is improved.
However, in the above technique, improvements of emission efficiency and modulation band may be able to be achieved insufficiently.
More specifically, consider that the number of quantum wells is excessively increased. In this situation, when holes 22 are injected from p-type cladding layer 2 into valence band well layers 23, holes 22 may not reach valence band well layer 23 because it is distant from p-type cladding layer 2 (i.e., on a light confinement layer 5 side). This is because holes 22 have effective masses. Therefore, hole concentrations in valence band well layers 23 are not uniform, and the effect of the multiple quantum wells is deteriorated.
Furthermore, even though modulation-doping is performed to only barrier layer 24 of the multiple quantum wells of the active layer to manufacture a semiconductor layer, p-type or n-type impurities which should only be in the barrier layer 24 actually diffuse to the well layer due to the thermal history of epitaxial growth or the like in the manufacture. For this reason, the impurity deteriorates crystallinity of the well layer serving as a light-emitting section and increase the nonluminous recombination ratio, so that the improvement effect is suppressed. This problem becomes significant as the concentration of added impurity becomes high and as the degree of diffusion of Zn or the like is high.
It is an object of the present invention to sufficiently improve emission efficiency and a modulation band in a semiconductor laser in which modulation-doping is performed to an active layer of a multiple quantum well structure.
A semiconductor laser of the present invention has an active layer formed between a p-type cladding layer and an n-type cladding layer. The active layer has multiple quantum wells having a plurality of barrier layers and well layers. P-type modulation-doping is performed to at least one of the barrier layers.
More specifically, a quantity of p-type modulation-dope of the barrier layer at a position close to the p-type cladding layer is smaller than that at a position close to the n-type cladding layer. For example, the quantity of p-type modulation-dope may be determined depending on a distance from the p-type cladding layer, or the quantities of p-type modulation-dope in barrier layers at a position close to the p-type cladding layer are gradually decreased. In addition, p-type modulation-doping may be performed at a first quantity of dope in at least one barrier layer at a position closer to the n-type cladding layer than a barrier layer at a predetermined position, and may be performed at a second quantity of dope smaller than the first quantity of dope in at least one barrier layer at a position closer to the p-type cladding layer than the barrier layer at the predetermined position. According to this feature, a differential gain and high-speed response can be improved while suppressing nonluminous recombination. At the same time, since the concentration of holes is high in a well layer distant from the p-type cladding layer, nonuniformity of carriers can also be improved. Note that p-type dopant of the p-type modulation-doping may be any one of Zn, Be, Cd, and C.
The semiconductor laser includes two light-reflecting films which have different reflectances such that the films are perpendicular to an end face opposing an active layer. According to the structure, light can be resonated and amplified. In particular, when the semiconductor laser further includes a diffraction grating, a fabry-Perot type laser which can extract a laser beam having a predetermined wavelength can be obtained. When a diffraction grating is arranged to overlap a layer surface of the active layer, a distribution feedback laser can be obtained. When a diffraction grating is arranged at a position distant from the layer surface of the active layer in the spreading direction of the layer surface, a Bragg reflector laser can be obtained.